In the image reading apparatuses that convert image information of a transparent original into electrical signals which are then read into a computer, film scanners such as those described in Japanese Laid-Open Patent Application Nos. 2000-324303 and 2001-189833 have been proposed, as apparatuses that use infrared light (which cannot be seen) to acquire dust and scratch mark information and to correct those portions of the visible image information that correspond to the locations of the dust and scratch marks.
FIG. 1 is a schematic diagram of such a conventional film scanner. FIG. 2 shows a conventional example of partial magnification. FIG. 3 shows a conventional example of read acquisition and image output.
As shown in FIG. 1, the film scanner is provided with an invisible (infrared) light source 1001 and a visible light source 1002. Image information from the film 1003 that is illuminated and read by one or the other of these two light sources is reflected by a mirror 1006, passed through a focusing lens 1007 and formed into an image on a line CCD 1008 aligned in a main scanning direction. The film 1003 is moved in a sub-scanning direction by a drive unit 1005 via a film mount 1004, with two-dimensional image information being read therefrom.
It will be noted that differences in wavelength between the invisible (infrared) light 1001 and the visible light 1002 cause the images thereof to form at different locations, so the apparatus is configured so that the focus is adjusted by the movement of an image pick-up unit 1009. The image pick-up unit 1009, which is moved by a focus drive unit 1010, is formed by combining the focusing lens 1007 and the line CCD 1008 into a single unit. The focus adjustment enables images based on both sources of light to be read when the focus is optimal. Additionally, because the two images are formed at different magnifications, the aforementioned Japanese Laid-Open Patent Application No. 2000-324303 also proposes correcting the overall magnifications by processing the invisible infrared image information using a signal processor 1011.
However, although the conventional apparatus described above can correct the overall magnification, as shown in FIG. 2, with a focusing optical system having partial magnification characteristics, correcting the overall magnification to fit the image periphery means that the magnification at the center of the image cannot be properly corrected. The result is that, as depicted in FIG. 3, the post-correction invisible (infrared) image (that is, the invisible infrared output image) shows partial distortion, with portions that do not match the magnification of the visible image.
The chief factors that cause differences in magnification of the infrared image and the visible image are differences in chromatic difference of magnification and comatic aberration due to the wavelength of the infrared light rays. With a focusing optical system such as that described above, correcting these differences in aberration requires a lens that uses expensive and hard-to-handle materials such as abnormal dispersion glass. The need for such correction has become especially apparent in recent years due to improvements in the read resolutions of image reading systems in particular, which tend to exaggerate differences in partial magnification.